Generally, an organic electroluminescence (EL) device includes an anode, a cathode, and at least one organic thin film layers which are interposed between the anode and the cathode. When applying a voltage between both electrodes, electrons are injected into an emission region from the cathode side and holes are injected into the emission region from the anode side. The injected electrons and holes are recombined in the emission region to generate an excited state. When the excited state returns to a ground state, light is emitted. Therefore, to obtain a high-efficiency organic EL device, it is important to develop a compound which efficiently transports electrons or holes into an emission region and facilitates the recombination of electron and hole.
Generally, when driving or storing an organic EL device in a high-temperature environment, various problems occur, for example, the emission color is changed, the emission efficiency is reduced, the driving voltage is increased, and the emission life is shortened. To eliminate these drawbacks, various hole transporting materials have been proposed, for example, Patent Document 1 discloses an aromatic amine derivative in which a N-carbazolyl group is directly bonded to a 9,9-diphenylfluorene skeleton, Patent Document 2 discloses an aromatic amine derivative in which a 3-carbazolyl group is directly bonded to a 9,9-dimethylfluorene skeleton, Patent Document 3 discloses an aromatic amine derivative in which a N-carbazolylphenyl group is bonded to a 9,9-diphenylfluorene skeleton via a nitrogen atom, and Patent Document 4 discloses an aromatic amine derivative in which a 3-carbazolyl group is bonded to a 9,9-diphenylfluorene skeleton via a nitrogen atom.
However, the aromatic amine derivatives disclosed in Patent Documents 1 to 4 are still insufficient for reducing the driving voltage and prolonging the lifetime. Therefore, a further improvement has been required.
Patent Document 5 proposes an aromatic amine derivative which includes a skeleton selected from a fluorene skeleton, a carbazole skeleton, a dibenzofuran skeleton, and a dibenzothiophene skeleton and teaches that an organic EL device in which the aromatic amine derivative is used as a material for an organic EL device, particularly as a hole transporting material, is capable of driving a low voltage and has a long lifetime. Patent Document 5 discloses a compound in which a diarylamino group is bonded to 2-position of a 9,9-diphenylfluorene skeleton. In the proposed compound, a carbazole skeleton, a dibenzofuran skeleton, or a dibenzothiophene skeleton must be bonded to a terminal end of one aryl group of the diarylamino group. In all the exemplary compounds having a 9,9-diphenylfluorene skeleton which are disclosed in Patent Document 5, a biphenylene group intervenes between the terminal groups and the nitrogen atom. However, a 2-diarylamino-9,9-diphenyl fluorene compound having such an intervening biphenylene group is insufficient in the emission efficiency when driving at a low voltage and also insufficient in the lifetime (see Examples 1-1 to 1-4 and Comparative Examples 1-1 to 1-2 described herein). Therefore, it has been required to develop a material for an organic EL device, particularly a hole transporting material, which can be synthesized easily and realize an organic EL device having a high efficiency when driving at a low voltage and long-lifetime.